There are known several types of self limiting electrical heating elements having geometrical configurations similar to those preferred in the invention. Such heating elements are known from, e.g., German patent No. 2,543,314 and the corresponding U.S. Pat. Nos. 4,177,376, 4,330,703, 4,543,474, and 4,654,511.
The heating elements described in said German patent DE-C2-2 543 314 relate in particular to heat recoverable articles. These articles are mostly used for sealing purposes such as covers for electrical components and cable joints. The heat recoverable article is arranged to be placed around the component or joint to be sealed, whereupon the article is connected to a power supply. The compositions and combinations of layers constituting the article are chosen such that the article is heated to a defined temperature at which the article shrinks and seals the electrical components or cable joint.
Similar heating elements are also described in U.S. Pat. No. 4,017,715 and EP-A1-0 237 228. The requirements of the elements described in U.S. Pat. No. 4,017,715 are that at room temperature the resistance in the ZTC layer is greater than the resistance exerted in the PTC layer.
From U.S. Pat. No. 4,689,475 there are known electrical heater devices which comprise at least one metal electrode and a conductive polymer in contact therewith, wherein the metal surface which contacts the conductive polymer has a roughened or otherwise treated to improve its adhesion to the conductive polymer. The metal electrode is preferably an electrodeposited foil. The conductive polymer preferably exhibits PTC behavior. The electrodes of these devices cover the entire area of the heater and there is only one current direction, namely that leading the shortest way through the PTC layer, from one metal foil to the other. While the described devices are claimed to include self-limiting heaters, their characteristics are substantially different from those obtained with the invention.
One of the objects of the invention is to provide a self regulating heating device, a property of which is its relative insensitivity to large variations in voltage at or near the thermal control temperature.
The invention will be described primarily in terms of composite devices wherein one component exhibits a positive temperature coefficient of resistance (PTC) and the other component exhibits essentially zero coefficient of resistance (ZTC) behavior.
A problem inherent in prior art devices depending solely on the variation of resistivity with temperature, has been that certain performance characteristics of the device were not always obtainable. For example, it is most desirable to maintain a given power output within a narrow control temperature range, and this temperature range does not always coincide with the anomaly temperature where most polymers exhibit their T.sub.s (switching temperature) and which is closely associated with the melting point in the case of crystalline polymers.
A feature of the invention is therefore to establish the control temperature of the device further removed from its crystalline melting point, since experience has shown that the closer a PTC component operates to its melting point, the less stable it is.